1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for producing it.
2. Discussion of Background
For hard magnetic disks used as high capacity recording media for computers, it has been desired to further increase the recording density in recent years. To accomplish the desired high recording density, it is usually very important to reduce media noises and to secure high coercivity of longitudinal magnetic thin films as recording media.
The reason why high coercivity is required, is that the length, a, of a magnetic transition region between magnetic domains constituting recording bits written by a magnetic head, depends on the in-plane coercivity H.sub.c of the magnetic film, as generally represented by a=2tM.sub.r f(S*)/H.sub.c, where t is the thickness of the magnetic film, M.sub.r is the remanence, and f(S*) indicates that a is a function of the coercive squareness S*, whereby the larger the value of S* is, the smaller f(S*) becomes. From this formula, it is evident that the larger the values of H.sub.c and S* are, the smaller the value of a becomes, whereby high density recording can be accomplished. The reason why high density magnetic recording is made possible by reducing the value of a, is that in the reproduction of a recorded information, voltages created in the head by magnetic domains written in the magnetic film are differentiated and taken out as output signals, and accordingly, the smaller the value of a is, the more rapidly the magnetization will be inverted, i.e. the narrower the width in time of the output pulses as the differentiated waveforms becomes.
The coercivities of magnetic films of small-size magnetic disks which are presently commonly used, are at a level of from 900 to 1,500 Oe. Further, the value of S* is desired to be reasonably high, at least 0.7. Here, the reason for the presence of the upper limit for the coercivity is that there is a limit in the magnetic field to be formed by the writing head to be used.
Recently, however, it has been made possible to fly the head at a lower height, and accordingly, it has been made possible to apply a high magnetic field intensity to the magnetic thin film even with the head having the same magnetic field intensity. Further, by an improvement of the magnetic film material used for the head itself or by the precision processing of the coil portion due to the progress in fine processing, it has been made possible to further increase the magnetic field created by the head. Accordingly, by a combination of such a technology of the head and the low flying height of the head, it should be possible to prepare a magnetic recording medium with a higher recording density by increasing the coercivity while maintaining the coercive squareness of the magnetic thin film at a reasonably high level of at least 0.7.
The magnetic thin film materials presently commonly employed for in-plane magnetic recording media include a CoPt-based alloy such as CoNiPt, CoNiCrPt or CoCrPt and a CoCr-based alloy such as CoNiCr Or CoCrTa which is used in combination with a Cr underlayer. Their coercivities usually have an upper limit at a level of 1,800 Oe at a remanence of 3.0.times.10.sup.-3 emu/cm.sup.2 which is usually required for magnetic recording media for an inductive head. With CoPt-based magnetic thin films among them, it is known that a high coercivity can be accomplished by increasing the Pt concentration to a level of from 20 to 25 atomic %, as disclosed in e.g. IEEE Trans. Magn. MAG-19 (1983) 1514, J. Appl. Phys. 54 (1983) 7089, or IEEE Trans. Magn. MAG-19 (1983) 1638. However, such a method is not practically useful, since it involves a substantial increase of costs. Further, incorporation of such a large amount of Pt element tends to lead to a substantial decrease in the remanence, such being undesirable from the viewpoint of signal to noise ratio at the reproduction process.
Accordingly, for a practical industrial application, the high coercivity must be accomplished with a low Pt concentration at a level of about 10 atomic % which is feasible from the viewpoint of costs. As such an attempt, it has been reported that a high coercivity at a level of at least 2,000 Oe has been accomplished by forming a CoCrPtB film (Pt: 6.5 atomic %) having B incorporated, on a Cr underlayer (14th Meeting of Japan Applied Magnetic Association, 8 pB-18 (1990)). However, the production process requires a substrate temperature as high as 280.degree. C. and a substrate bias of -300 V and thus has substantial yield problems yet to be solved for mass production. Further, Japanese Unexamined Patent Publication No. 84723/1991 discloses that a magnetic film having a high coercivity at a level of at least 2,000 Oe was prepared by incorporating an element such as P, Si, Ge, B, Ga, Al, In, Sn or Sb to CoPt and sputtering the material in an Ar gas containing oxygen. However, the magnetic film prepared in such a manner has a serious practical problem such that both the squareness and the coercive squareness are less than 0.7, and it can not be regarded as suitable as a high density recording medium.
Further, in designing a practical hard magnetic drive, an optimum value exists for the coercivity for the read and write characteristics, particularly for the overwrite characteristics, by the combination with a head, and it is necessary to adjust the coercivity of the magnetic film. Accordingly, the coercivity must be adjusted to a high level and must be easily adjusted for mass production without substantially changing other magnetic properties.
Further, to accomplish high density magnetic recording, it is very important for a magnetic recording medium that the noise attributable to the medium during the reproduction of a recorded information is low. The media noise of thin film media are usually dominantly caused by the irregularities in the domain structure of the magnetic transition boundary regions i.e. by the formation of so-called zigzag domains, which is largely influenced by the crystal grain structure of the magnetic thin film (J. Appl. Phys., 63, 3248 (1988)).
Accordingly, to reduce the media noise, it is necessary to study the alloy composition of the magnetic film and to control the sputtering process to modify and control the crystal grain structure of the magnetic thin film. It is known that the media noise can be reduced by using a NiP sputtering underlayer for a magnetic film of CoPt-based alloy (U.S. Pat. No. 4,786,564). In this proposal, the crystal grain structure of the NiP underlayer is effectively used to control the crystal grain structure of the magnetic film.
However, as a result of a study by the present inventors, it has been found that to reduce the noise to a level required for future high density magnetic recording, this method alone is inadequate, and it is necessary to further reduce the media noise by improving the magnetic film material and utilizing the synergistic effect of the underlayer and the magnetic film material. Thus, if the above-mentioned improvements for the high coercivity and reduction of the media noise can be accomplished with a CoPt-based magnetic film using a NiP sputtering film as the underlayer, it will be possible to accomplish magnetic recording at a higher density.